1. Field of the Invention
The present invention relates to a noise canceling apparatus and a noise canceling method that are applied to reduce noise inside vehicles for example.
2. Description of the Related Art
Recently, the popularization of portable music reproducing apparatuses has been increasing the occasions of listening music out of doors. For example, listening to music in vehicles, such as trains and buses, the volume of music being reproduced must often be increased to overcome the noise level in such environments. Consequently, the sound leaked from the portable music reproducing apparatus increases, possibly causing trouble to the people around. In order to solve this problem, noise canceling headphones have recently been put in practical use that has noise canceling capabilities of reducing surrounding noise.
For a noise canceling headphone, that based on a feedback scheme is known, for example. In this scheme, a microphone is arranged inside a headphone and the phase of a signal picked up by the microphone as noise (hereafter, this signal will be appropriately referred to as a noise signal) is inverted to generate a signal of inverse phase. This signal having the phase inverse to the phase of noise (hereafter, this signal will be appropriately referred to as a noise cancel signal) is supplied to a driver unit of the headphone, thereby reducing the noise heard by the user.
With the noise canceling headphone based on the feedback scheme, a microphone 101 and a loudspeaker 102 are arranged in the proximity of observation point P0 that is assumed to the position of user ear as shown in FIG. 9, for example. When a noise signal from a noise source is picked up by the microphone 101, a noise cancel signal having a phase that is inverse to the phase of the picked up noise signal is outputted from the loudspeaker 102. At observation point P0, a signal obtained by synthesizing these noise signal and noise cancel signal is observed.
For example, as shown in FIG. 10A, a noise signal with amplitude level “1” is observed at observation point P0. If this noise signal is picked up by the microphone 101, a noise cancel signal with amplitude level “0.9” and phase reverse to the noise signal is outputted from the loudspeaker 102 so as to cancel the noise signal picked up by the microphone 101 as shown in FIG. 10B. It should be noted that, in what follows, the amplitude level and the phase of each noise cancel signal are represented by a number and a sign, respectively. For example, a signal having amplitude level “0.9” and inverted in phase is simply written as “a signal of “−0.9.””
At observation point P0, a noise signal “1” shown in FIG. 10A and a noise cancel signal of “−0.9” shown in FIG. 10B are synthesized and a noise signal attenuated to “0.1” is observed as shown in FIG. 10C. Therefore, the effect of noise cancellation is 1/10 (=−20 dB).
A related-art noise canceling apparatus 100 has a microphone 101, a loudspeaker 102, an analog circuit 103, and an amplifier 104. The microphone 101 converts ambient sound picked up as noise into a noise signal and supplies the noise signal to the analog circuit 103. The analog circuit 103 executes predetermined signal processing on the supplied noise signal to generate a noise cancel signal with phase inverted. The noise cancel signal outputted from the analog circuit 103 is amplified by the amplifier 104 to a predetermined level to be supplied to the loudspeaker 102 to be outputted therefrom.
As described above, the related-art noise canceling method based on the feedback scheme is configured that the effect of noise cancellation is maximized at the position of microphone. Hence, if the microphone and the loudspeaker are arranged in the proximity of the observation point, the effect of noise cancellation can be sufficiently obtained at the observation point.
Also, recently, a method is proposed in which noise canceling capabilities are installed on a car-mounted music reproducing apparatus to reduce noise inside the car. Like the above-mentioned noise canceling headphone, the noise cancellation by the car-mounted music reproducing apparatus is also executed by placing the microphone in the proximity of the loudspeaker mounted on a door of the car. Then, a noise signal detected by the microphone is fed back to the amplifier for driving the loudspeaker and a noise cancel signal inverted in phase to the noise signal is outputted from the loudspeaker, reducing the noise in the proximity of the microphone. For example, Japanese Patent Laid-Open No. Hei 10-333687 describes a noise reducing apparatus for reducing in-car noise.